Recently, lithium ion secondary batteries, which have high energy density and are also clean, have attracted great attention and high expectations.
Lithium ion secondary batteries, including a negative electrode containing a carbon material capable of doping and undoping lithium and a positive electrode containing a lithium complex oxide such as lithium cobalt oxide or lithium nickel oxide, have been actively developed in recent years. With optimization of the design capacity of the positive electrode/negative electrode, these lithium ion secondary batteries can eliminate growth of lithium dentrites which can be observed in batteries using lithium metals, can provide excellent properties such as low self-discharge, excellent cycle characteristics, and safety properties, as well as excellent low-temperature characteristics, load characteristics, or high-speed charging properties. As such, there are increased expectations for the lithium ion secondary batteries. At the same time, the lithium ion secondary batteries have been currently put into practical use as a power source for portable devices such as laptop computers, word processors, camera-integrated VTR, liquid crystal televisions, cellular phones, and so on.
Further, in addition to the use for these compact consumer products, technical developments of lithium ion secondary batteries as power storage devices or large-capacity and large-size batteries for electric vehicles and other applications have been accelerated. In particular, the development of lithium ion secondary batteries for hybrid electric vehicles has been rapidly pursued.
In the process of manufacturing electrodes in lithium ion secondary batteries, an electrode mixture having flowability is applied to a metal leaf which is a current collector and is then dried, thereby manufacturing the electrodes (positive electrode and negative electrode). The electrode mixture composition is composed of an active material which directly contributes to a battery reaction (a charge/discharge reaction), a conductive agent for supporting this battery reaction, a binder for binding these materials, and a diluted solvent and a thickening agent for achieving uniform mixture and application of these materials, or the like.
The drying process performed after application is aimed at evaporating especially the diluted solvent in the electrode mixture which does not contribute to the battery reaction. At this time, however, due to convection of the binder within the electrode mixture, there is a possibility that the binder fails to be distributed uniformly in the coating and is localized in the surface of the coating (electrode surface). Such an uneven distribution or localization of the binder to the electrode surface would cause problems in manufacturing, including an increase in the resistance in the electrode surface to prevent smooth progress of the charge/discharge reaction or result in peeling of the electrode mixture from the current collector, and so on.
Accordingly, in order to suppress the uneven distribution or localization of the binder, Patent Document 1, for example, discloses a method of drying an electrode mixture of a negative electrode under control of the rate of removing moisture, during the drying process.
Further, Patent Document 2, for example, discloses a method in which, during the drying process, drying is performed under the condition that the temperature of hot air supplied from above the electrode is 90° C. or less and the temperature of hot air supplied from below the electrode is 110° C. or more.
Also, Patent Document 3, for example, discloses a method of applying an electrode mixture containing carboxymethyl-cellulose and a pH adjustor and a having a pH of 5 or greater and 9 or less onto a current collector, and drying the electrode mixture.
In addition, Patent Document 4, for example, discloses a method of applying an electrode mixture containing vinyl polymer onto a current collector and drying the electrode mixture.